Transducer control



p 5 H. El VAN VALKENBURG ET AL 2,651,612

TRANSDUCER CONTROL 2 Sheets-Sheet 1 Filed June 12, 1952 a N m my m mm Z 0 P R 1 R Em 6M 5M m 5 6 Wm Wm 6 6 2 1 2 2 D R 2 o f m W W N m v w k 0 6 I. 0 f f 3 F W 0 =1 V/aRAro/F L [NI 'ENTORS. HOWARD E. VANVALKENBURQ EDWARD c. COOK gw m f ATTORNEY P 1953 H. E. VAN VALKENBURG ET AL TRANSDUCER CONTROL 2 'sheets-sheetz Filed June 12, 1952 INVENTOR5.-

HOWARD E. VANVALKENBURG EDWARD G. COOK ATTORNEY IIL FIIIL Fllllll IIIL Patented Sept. 1, 1953 TRANSDUCER CONTROL Application June 12, 1952, Serial No. 293,228

4 Claims.

This invention relates to pulse generators and has for its principal object the provision of a method and means for causing a transducer to enerate a wave train of short duration. The usual wave train which is generated by a transducer, such as a piezo-electric quartz crystal, is characterized by an exponential amplitude demay. It is highly desirable in many applications that these mechanical vibrations generated by the quartz crystal be stopped sharply after a desired number of oscillations. For example, in the testing of objects by transmitting ultrasonic energy into the object and measuring the time interval between transmission of the energy and reception of its reflection from the first reflecting surface within the object, it will be seen that unless the Wave train generated by the transducer is short and sharply cut off it is not possible to inspect the region of the object close to the entering surface. This is due to the fact that if the wave train is relatively long, reflection of the initial portion of the wave train from a reflecting surface near the entering surface of the object will be received before the transmission of the remainder of the wave train has ended. In an effort to accomplish the above result several methods have heretofore been employed. One of these consisted in mechanically loading the faces of the crystal, but this did not stop the oscillations soon enough. Another method which has been proposed is to apply a critically damped electric energizing Wave train to the transducer crystal. This resulted in a sharp electrical energization of the crystal for a short period, but nevertheless caused the crystal to generate after this energization an exponentially decaying vibration, although it did reduce the duration of this vibration. g

It is the principal object of this invention to provide a method and means for terminating the vibration generated by the transmitting face of the quartz crystal after any desired number of oscillations which may be predetermined at will.

Further objects and advantages of this invention will become apparent in the following detailed description thereof.

In the accompanying drawings,

Fig. l is a block diagram embodying one form of this invention.

Fig. 2 is a view similar to Fig. 1 but disclosing the wiring details.

Fig. 3 is a diagrammatic representation of a modificationof the Fig. 2 form of the invention.

Figs. 4A, B, and C are a series of graphs illustrating the principle of the invention,

Referring first to Fig. 4A it will be seen that the typical Wave train Pl generated by the transducer is characterized by an exponential decay wherein the oscillations of the vibration gradually lose amplitude from maximum to zero in a time interval ii to is. This is the type of oscillation which results normally when a quartz crystal is energized by a short electric wave train. As stated in the introduction hereto, the quartz crystal H3 will transmit the mechanical vibrations corresponding to the oscillations of such wave train into an object H with which the crystal is in engagement. The wave train will travel into the object H until it strikes a reflecting surface, which may be either the rear surface 12 or an internal reflecting surface such as defect D, whereupon the wave train will be returned to the crystal. It will be seen that if the transmitted Wave train is of relatively long duration, reflections will start coming back from a reflecting surface such as D lying near the entering surface l5 before the transmitted wave train has terminated at the crystal. This will render it difiicult to detect returning reflections, and therefore an instrument such as the ultrasonic reflectoscope which depends upon indicating the time interval between the transmission and reception of a Wave train could not operate efficiently to detect such defects. It is therefore desirable that the transmitted wave train be as short as possible, while at the same time having the oscillations of sufiicient amplitude to provide the necessary power for traversing the object.

To accomplish the above it is desirable to provide a system which will, after a brief time interval 151 to 152, eliminate further oscillations of the transmitting face of the crystal, as shown in Fig. 40. One method of accomplishing this result would be to apply to the crystal electrical energization, which would tend to vibrate the transmitting face of the crystal in a direction opposite to the direction in which said crystal face is being vibrated by the original electrical wave train at time 152. Such an electrical impulse would cause the transducer to generate an oscillation P2 as disclosed in Fig. 43 wherein it will be seen that the wave train applied to the crystal tends to produce, for example, a cycle l6 of similar shape and opposite sign to the cycle I! previously generated by the transmitting face of the crystal. Since the pulse B tends to vibrate the transmitting face of the crystal with a force equal and opposite to the force which energizing wave train A tends to vibrate said crystal face beginning at time t2, it will be seen that all further tendency of the transmitting face of the crystal to vibrate after time t: is substantially eliminated. The problem therefore is to provide a method and means whereby after the time interval t to te a wave train is applied to the crystal which tends to vibrate the transmitting face of the crystal equally in magnitude but opposite in direction to the tendency of said crystal face to vibrate in response to the wave train which is previously applied to the crystal at time ii.

In order to accomplish the foregoing result we provide the arrangement disclosed in block form in Fig. 1 and in detail in Fig. 2. In this form of the invention a multivibrator 20 is energized from a suitable source (here shown as 300 volts) and the wave train thus generated is applied to the grids of two normally conductive trigger tubes 2! and 2!. The outputs of the trigger tubes are differentiated by differentiators 22 and 22' in the grid circuits of two cathode followers 25 and 25', which are in. turn in the grid circuits of two thyratron tubes T and T of wave train generators P and P. Each difierentiator comprises a capacitor 53, I3 and a resistor M, M, and only the positive pulse outputs of the diiferentiators are effective to trip the follower tubes and the thyratron tubes. The differentiator output in the circuit of the auxiliary thyratron T acts through a time delay which may comprise a capacitor 26 and a resistor 21, one of which is adjustable. By means of this time delay the trigger of the auxiliary thyratron may be delayed for any desired time interval 151 to is after discharge of thyratron T.

When the main thyratron T fires it will permit a capacitor 33 to discharge through a pulse shaping circuit 3| whose output is applied to an amplifier tube 32 whose output is applied to the crystal Hi. This wave train will set the transmitting face of the crystal vibrating to cause it to transmit a mechanical wave train into object I I, said wave train being of the exponential decay type P shown in Fig. 4A. After a time delay set as described hcreinbefore to correspond to the time interval t1 to t2, the auxiliary thyratron T will fire to permit a capacitor 39 to discharge into pulse shaping circuit 3i whose output is applied to amplifier tube 32' from the output of which the wave train is applied to the transmitting face of the crystal !0. The two tubes 32 and 32' have their outputs connected .at 33 and therefore serve to isolate the two pulse shaping circuits from each other so that each pulse shaping circuit may act independently on the crystal it without affecting the other pulse shaping circuit. The two tubes 32 and 32' as connected therefore serve as .pulse separation circuits to permit each wave train to act independently of the other on the transmitting face of the crystal.

Since the mechanical wave train P2 begins to be generated only after time interval 151 to its, it will be seen that if it can be made of the same shape but opposite in phase to the mechanical wave train which is generated by the transmitting face of the crystal at the particular instant, the transmitting face of the crystal will be stopped from vibrating. Therefore the pulse shaping circuit 3| is provided with an inductance coil 35, adjustable resistor 36, and adjustable capacitor 31, by which adjustments the frequency and the wave form of the electrical pulse may be controlled to provide whatever shape of mechanical pulse is necessary to be equal and opposite to the oscillation existing on the transmitting face of the crystal by reason of electrical pulse P at that particular instant. In this manner, by applying two separate electrical pulses to the transmitting face of the crystal after a time delay, and the later pulse shaped so that the wave generated by the transmitting face of the crystal is equal and opposite to the oscillation of the crystal face as a result of the first electrical pulse, the crystal face may be brought to a complete stop.

In a modified form of the invention disclosed in Fig. 3 the same result may be achieved by eliminating all of the circuit after points A and B inFig. 2 and causing the outputs of the shaping circuits to be applied directly to independent and separate energizing electrodes 40 and 4! of the single crystal l0. By this modification the necessity of employing separating tubes 32 and 32 is eliminated and the same result of com pletely separating the two electrical wave trains acting on. the. transmitting face of the crystal is achieved.

Having. describeclour invention, what we claim and desire to. secure by Letters Patent is:

.1. The method. .of producing by a crystal transducer a wave train of mechanical energy of a controlled number of cycles, which consists in generating a voltage wave train, applying the Wave train to the transducer to cause the transducer to vibrate.mechanicallmgenerating a second voltage wave train which is electrically independent of said first wa-ve train, shaping the second wave train, and applying the .second wave train to the transducer a predetermined time interval after the first wave train has been. applied to the transducer to cause mechanical vibrations produced by the face .of the transducer equal and opposite to the mechanical vibrations existing at the face of the transducer at the instant of appli cation of the second wave train.

2. The method. of producing by a crystal transducer havingv a plurality of energizing electrodes a wave train of mechanical energy of a controlled number of cycles, which consists in generating a voltage wave train, applying the wave train to one of the transducer electrodes to cause the transducer to vibrate mechanically, generating a second voltage. wave train. which is electrically independent of said first wave train, shaping the second wave train, and applying the second wave train to another transducer electrode a predetermined time interval .after the first wave train has been applied to the first transducer electrode. to cause mechanical vibrations produced by the face of the transducer equal and opposite to the mechanical vibrations existing at the face of the transducer at the instant of application of the second wave train.

3. A devicev for producing a wave. train of ultrasonic energy -.of acontrolled number of cycles, comprising .a crystal transducer, a voltage wave train vgenerator, asecond voltage wave train generator electrically independent of the first generator, means for energizing the wave train generators, delay meansfor delaying energization of the second wavetrain generator,..means for shaping the wave train generated by the second wave train generator after a predetermined time delay means whereby the generators apply their wave trains to the transducer to cause mechanical vibrations generated by .the..-face of. the transducer equal and opposite to the mechanical vibrations existing at the .face of the transducer at the instant .of application of the second voltage wave train.

1. A device for producing an ultrasonic wave train of energy of a controlled number of cycles, comprising a crystal transducer having a plurality of energizing electrodes, a voltage wave train generator, a second voltage wave train generator electrically independent of the first generator, means for energizing the generators, delay means for delaying energization of the second Wave train generator, means whereby the first generator applies its wave train to one of said transducer energizing electrodes, means for shaping the wave train generated by the second wave train generator after a predetermined time delay means whereby the second generator applies its wave train to another of the transducer energiz- HOWARD E. VAN VALKENBURG. EDWARD G. COOK.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Mason Feb. 25, 1947 Number 

